The JPL scientist in charge of tracking incoming asteroids tells us if we should be worried.

The subtitle of your book – Finding Them Before They Find Us – refers to this role that asteroids play in the destruction of life. How worried should we be that one of these things is going to come hurtling toward us any time soon?

I don’t think anyone should lose any sleep over this. But as we noted earlier, until the 1990s people just weren’t looking. It’s not as if these things weren’t zipping by every two weeks – they were – we just never saw them. So we were oblivious to the possible danger. Now NASA has been tasked by Congress to actually find and track them. The first goal was to find 90% of the near-Earth asteroids larger than a kilometer and track them, and we’ve done that. The kilometer was selected because the first step was to find the objects that could cause global problems. We’ve found almost 95% of that population so far, and none of them present a threat. Then Congress said, “NASA, good job, now find 90 percent of near-Earth objects 140 meters and larger.” After a few years, we’re 40 percent to reaching that goal. [The diameter] 140 meters was selected because that’s roughly the size of an object hitting the ocean that would cause a tsunami, or hitting land that would cause widespread regional devastation. We’re already starting to move onto the smaller ones, [because] anything above about 30 meters could cause a real problem; anything less than that, it wouldn’t likely survive through Earth’s atmosphere, it would just be a compressive fireball.

How do you make that calculation? If you’ve found approximately 40% of the near-Earth asteroids 140 meters and bigger, how do you know what the total number is in order to get a percentage?

It’s all done statistically. If our NASA-supported telescopes are searching every night for new near-Earth objects, let’s say over a ten-year period they find 900 objects larger than a kilometer. At the end of that ten-year period, it turns out that for every 10 objects they find, nine of them have already been discovered. Then you would say, alright, we’re at a point where we’ve found 90 percent of the population already, and if we’ve found 900 of them, the total population is likely to be 1000.

What is the best way to find near-Earth asteroids?

The ground-based optical observations are finding a lot of these objects. On the other hand, if you’re asking what way would be most efficient, then you would need a near-infrared telescope in space. Because these asteroids are dark, they absorb sunlight and radiate heat, mostly in the infrared, so they’re much easier to see in the infrared than in the optical. Ideally you’d like to have an infrared detector at the back end of a good-sized telescope, in either Earth orbit or one that’s not too dissimilar to the orbit of Venus. That’s the [B612 Foundation] Sentinel approach, and that would be the ideal technique for finding these objects.

It was found by a couple of Russians at an observatory, International Scientific Optical Network, that’s what the name ISON stands for. This comet is already fairly bright and its out near Jupiter, so if we extrapolate its brightness as it comes in very close to the sun on Thanksgiving Day of this year, it has the potential to be very bright in the morning sky, or actually in the evening sky after sunset. Having said that, I’m old enough to remember Comet Kohoutek back in 1973; we predicted that would be the comet of the century and it turned out to be a bit of a fizzle. Comet brightness and behaviors are notoriously difficult to predict. But, if this comet behaves itself and acts like most comets it will put on a pretty good celestial show come this November.

What seems interesting, and tying it back to your book, is that they only just found it a few months ago. There are still these surprises out there in the sky, so this comet seems like a good argument for why we need to keep looking all the time.

That’s true. Comets, some of them get cut from the Oort cloud, which is at the very limit of our solar system, at 100,000 times the distance between the sun and the Earth. It takes millions of years for those objects to get from the Oort cloud to the inner solar system. ISON is a comet from the Oort cloud, so it’s almost certainly on its first return to the inner solar system. It’s scientifically interesting because it really hasn’t evolved much; it’s been out in the deep freeze of this Oort cloud for so long, retaining the percentage of ices and dust that formed some 4.6 billion years ago. With comets that have been around the sun several times, like Halley or other periodic comets, the volatile ices like carbon monoxide or carbon dioxide usually get depleted, and you’re left pretty much just with water ice. But these new [comets], they come in with their entire suite of volatiles – carbon monoxide, carbon dioxide, methane, ammonia, water – and so they’re the ones that are really the most interesting. I think that’s why astronomers are particularly excited about this comet.